Syria's government insists it doesn't have chemical weapons, and wouldn't use them against its own people if it did, while the Syrian opposition says it neither has such munitions nor the means to make them.

The prospect that these could potentially be deployed by an increasingly beleaguered regime has made many observers anxious -- and has been cited as a "red line" for robust action by the United States.

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U.S. eyes 'red line' on Syria

Few munitions evoke as much fear as chemical weapons. And unlike nuclear weapons, they are relatively inexpensive to develop and stockpile.

This lends them a disproportionate importance for Syria and the region, analysts say.

"In the Middle East, chemical weapons have been seen as a possible counter to Israel's nuclear weapons," said Dr. Susan B. Martin, of the Department of War Studies at King's College London.

"They are seen as a possible strategic deterrent," she said, "and they are cheaper and easier to have than nuclear weapons."

Several countries in the Middle East have refused to sign the Chemical Weapons Convention until Israel signs the Nuclear Non-Proliferation Treaty.

Dina Esfandiary, a research associate at the International Institute for Strategic Studies, says Syria in particular embraced a chemical weapons program as a way to bolster its strategic strength despite economic weaknesses, especially after Israel imposed a series of humiliating military defeats on the Arab world.

"The best way to operate asymmetrically was for Syria to have its chemical weapons program," she said.

The Syrians started working on research and development of such munitions in the 1970s and have continued to invest in the program since, said Esfandiary.

It's difficult to quantify how large its stockpiles are, but experts believe that Syria has the largest program in the Middle East and the fourth largest in the world.

"The important thing in terms of Syria is that it requires very little investment, very little technology and is very easy to conceal," said Esfandiary.

Close to a century has passed since their devastating deployment by both sides on the battlefields of the First World War prompted widespread revulsion. The United Nations Office for Disarmament Affairs estimates that nearly 100,000 deaths resulted from their use then.

Efforts have been made since 1918 to stamp them out -- through the 1925 Geneva Protocol, which bans their use, and the 1997 Convention on Chemical Weapons, which bans their use, development, production and transfer -- but still some nations stockpile them.

Much of the work in developing and manufacturing chemical weapons occurred during the Cold War period.

The UNODA estimates that by the 1970s and '80s, 25 states were developing chemical weapons capabilities, and that since the end of the First World War, such munitions have caused more than 1 million casualties globally.

Unlike much of the rest of the world, the Middle East has witnessed the use of such weapons in recent decades -- by Egypt against Yemen in 1963, as well as by Iraq against Iran and its own Kurds in the 1980s, Martin said.

Perhaps the most notorious instance is their use by Saddam Hussein's regime against civilians in the Iraqi Kurdish city of Halabja.

More than 5,000 people died in only 20 minutes and another 10,000 were severely injured in the attack, according to the U.S. State Department.

One reason for the high toll was the way the Iraqi forces deployed their arms, said Patricia Lewis, a fellow at the UK-based think tank Chatham House.

They first used conventional weapons to blow out the windows and doors of the homes where civilians were taking shelter, often in cellars. When they then fired chemical weapons, the toxic gases seeped in and often pooled in the cellars, proving even more deadly, she said.

In the years since the attack, civilians who survived have suffered much higher rates of serious diseases because of the toxic chemicals in the weapons, the UNODA said.

Iraq used unconventional weapons despite it being party to the 1925 Geneva Protocol.

Currently, 188 nations, representing the vast bulk of the world's population, have signed up to the Convention on Chemical Weapons, which is overseen by the Organization for the Prohibition of Chemical Weapons.

Syria is not one of them.

What impact could chemical weapons have if used now?

According to Esfandiary, chemical weapons' utility is "quite limited," as they are more of a deterrent than a real battlefield or tactical weapon.

"If you shoot a missile at a a population center, you can be fairly certain that anyone it hits will die," she said. "Chemical weapons use is not as clear cut as that -- it depends on topography, weather, how you deliver the chemical weapons, and you can't always be clear it will cause maximum casualty."

Their real value is in their psychological power, she said. "It's a fantastic weapon of fear."

They can cause economic damage too if their use contaminates agricultural areas or water, making them also "a good weapon of disruption," she said.

Martin, of King's College London, points out that the threat of chemical weapons being deployed against foreign forces by Hussein did not stop international intervention in Iraq in 1991 or 2003.

The Iraqi army had experience in targeting and using such weapons in war, she added, making it a greater threat in this respect than Syrian forces today.

But Esfandiary argues that although Syrian forces are not believed to have used chemical weapons, they are "very much present in Syria's military doctrine."

She added: "The Syrian military has been very well trained in their use and deployment, so if anyone was to use them and use them successfully, it's Syria."

Modern armies are equipped to cope with such threats. Civilians, or ill-equipped rebel forces, are not.

If such weapons are fired in confined spaces, such as buildings, their effects are far more deadly than in the open air, said Lewis, of Chatham House.

An added danger is that chemical weapons have a long shelf life. Even if not usable as munitions, the chemicals can still present a threat decades on.

"One of the issues which is still being dealt with is munitions that are left over from the Second World War," said Martin.

How can you tell if chemical weapons have been used?

It's difficult to determine if chemical weapons have been deployed, unless you can recover a munition that still has traces of agent on it, said Martin.

Some conventional weapons or legal crowd control can also release smoke that causes respiratory problems, a common symptom of chemical weapons exposure.

This seems to have been the case in Homs last December, where the Syrian government was accused of using chemical weapons against civilians.

A U.S. State Department investigation subsequently concluded that the Syrian army did not use chemical munitions but apparently misused a riot-control gas in the attack, which Syrian doctors and activists said killed six people and left dozens suffering from respiratory, nerve and gastrointestinal ailments.

In order to be sure of what happened, it's key to get people on the ground quickly who can speak to any alleged attack victims, study medical records and analyze samples before they decompose, said Lewis.

The United Nations investigation team will be sent to Syria "as soon as practically possible," U.N. Secretary-General Ban Ki-moon said, but it's a complex mission that will not happen overnight.

U.S. officials remain to be convinced that the claims of chemical weapons use either in Khan al-Asal or in the rural Damascus suburb of Ateibeh are credible.

U.S. President Barack Obama and other American officials have said they are "deeply skeptical" of Syrian government claims that the opposition used chemical weapons.

Analysts are also "leaning hard away" from the notion that Syria used chemical weapons against its own people, a U.S. military official directly familiar with the preliminary analysis told CNN.

Pictures of the supposed victims of the chemical weapons attacks do not show the signs of burns or other symptoms that one might expect to see as a result of sarin, mustard gas or VX use, Esfandiary said.

What's to stop Syria from using its reputed arsenal?

The use of chemical weapons could be the final straw that pushes the international community to intervene directly in the two-year-long conflict.

After the latest claims, Obama reiterated his warning to Syria's government that it would be held accountable for the use of chemical weapons "or their transfer to terrorists."

He first warned last August that any sign of chemical weapons being moved around in large quantities or utilized would be "a red line for us."

The Russians, who have been staunch allies of Syrian President Bashar al-Assad, have indicated that the use of chemical weapons would be a step too far.

Ban also set out the U.N. position clearly, saying his announcement of a U.N. investigation "should serve as an unequivocal reminder that the use of chemical weapons is a crime against humanity."

The relative success of the Chemical Weapons Convention has put their possession and use well outside the norm, experts say. This stigma makes it easier for the United States and others to pressure al-Assad on this point.

However, the muted international response when his forces have used a variety of conventional weapons against his people may have emboldened him, said Esfandiary.

"The best way the West can react is to continue to make their 'red lines' absolutely clear," she said. "The danger is if you get to a situation where Assad really has got nothing to lose, then he really won't care. To put it simply, he won't use them until the very, very last minute."

Lewis, the Chatham House fellow, agrees that the stakes are too high for al-Assad's forces to use chemical weapons lightly, which adds to her skepticism of the rebels' latest claims that they were used near Damascus.

Could they fall into the wrong hands?

Reports have repeatedly surfaced in recent months of Syrian forces moving some of the chemical weapons inventories, possibly because of deteriorating security in the country.

This has raised fears the stockpiles could fall into the hands of al Qaeda-linked groups working with the opposition, should al-Assad's government fall.

Russian Foreign Minister Sergei Lavrov said in December that Syria had consolidated its chemical weapons into one of two locations in the face of ongoing rebel gains.

Esfandiary says she believes it's unlikely that any unconventional munitions have fallen into rebel hands so far.

"Keep in mind that this is Assad's most prized possession. How likely is it that he's going to hand over such important stockpiles to his enemies?" she said.

One disturbing possibility, though -- raised by the opposition -- is that the Syrian government would accuse rebel forces of using them in order to justify then retaliating in kind.

What does Syria say?

Syria had always denied having chemical weapons. But last July, then-Foreign Minister Jihad Makdissi confirmed for the first time that Damascus has "unconventional" weapons, but vowed they "would never be used against civilians or against the Syrian people during this crisis at any circumstance."

"All the stocks of these weapons that the Syrian Arab Republic possesses are monitored and guarded by the Syrian army. These weapons are meant to be used only and strictly in the event of external aggression against the Syrian Arab Republic," Makdissi said. He has since fled to the United States as a refugee, according to the U.S. envoy to Syria, Robert Ford.

In a letter to the U.N. secretary-general in December, Syria said the United States falsely accused it of using chemical weapons.

"What raises concerns ... is our serious fear that some of the countries backing terrorism and terrorists might provide the armed terrorist groups with chemical weapons and claim that it was the Syrian government that used the weapons," the state-run news agency SANA reported.

It was its formal request to the U.N. secretary-general that prompted the promise of an independent investigation into their alleged use.